Energy storage using large superconducting magnets has been proposed for leveling daily load requirements on electrical utility systems. Excess energy generated during off-peak hours can be stored and later returned to the power grid during high demand periods. By connecting the superconducting energy storage magnet to the power system with a bridge-type inverter, it is possible to obtain very efficient energy transfer between the storage magnet and the power system, as more fully described in U.S. Pat. No. 4,122,512 to Peterson, et al.
The large energy storage magnets proposed for storing sufficient energy to allow load leveling on a power grid utilize multiple turns of composite normal and superconducting material. The current flowing in the turns of the magnet naturally produces a net magnetic field and any conductor in the field will experience a force at each point on the conductor oriented at right angles to the current and the magnetic field. Since superconducting magnets of the size proposed for electrical system energy storage will conduct extremely large currents and will generate strong magnetic fields, the forces experienced by the conductors will be very large. If the turns of the magnet coil were formed as conventional circular turns, and were unsupported, the tension at any cross-section in the conductor would be equal to BIR, where B is the component of the magnetic field experienced by the conductor perpendicular to the plane of the conductor (axial magnetic field), I is the current in the conductor, and R is the radius of curvature of the turn. In the large energy storage magnets under consideration, all of these factors will be very large, e.g., several hundred thousand amperes will be conducted in a field of several teslas in a solenoid magnet having a radius which may be several hundred meters. Since no conductor by itself could possibly withstand the forces that would be exerted on the conductor under these conditions, an external support structure capable of resisting the large loads imposed on the conductor is thus necessary. However, substantial practical difficulties are encountered in supporting the superconducting magnet because of the supercooled conditions under which the magnets must be operated. The support structure must not add a significant thermal load on the cooling system and must be capable of adjusting to the expansions and contractions encountered during the initial cooldown of the system and any subsequent heating and cooling cycles.
One approach to the problem of adequately supporting a superconducting magnet is shown in the U.S. Pat. No. 3,980,981 to Boom, et al. The structure disclosed in that patent includes a rippled composite superconducting-normal conductor which is laid out in a single layer of turns disposed in a trench formed in the ground. Each ripple in the conductor lies in a plane normal to the net magnetic field experienced by that conductor. The outward force on the conductor is opposed by support columns which engage the conductor at its innermost portions between the ripples. The supporting columns extend radially to an outer support wall which may be formed in bedrock. The columns can be made of insulating material so that the necessary thermal shielding Dewar is accommodated around the conductor with minimal interference from the radial support members.
The single layer magnet coil disclosed in U.S. Pat. No. 3,980,981 has several advantages, including ease of maintenance since both sides of the conductor are readily accessible, a simple construction for the conductor, reduced stresses resulting from the rippling in the conductor, accessibility of both sides of the conductor with a mechanical shorting switch to protect against failure of the cooling system, ability to surround the conductor with superfluid helium for maximum cooling efficiency, and low voltage difference levels between turns in the magnet coil. Despite the advantages of the single layer design, all of the current circulating in the superconducting coil must be carried by a single conductor. For magnet designs under consideration for power system load leveling, a current capacity of 750,000 amperes or more would be carried by the single conductor. Additionally, the resultant of the forces on the rippled conductor will be substantially radial, so that a very strong and stable outer support mass is required to carry the loads that will be imposed when the superconductor is carrying current. If the conductor is buried in and surrounded by bedrock, which is intended to carry these radial forces, the bedrock must have reasonably good structural integrity and be stable over time.